As is known in the art, microwave power amplifiers, such as, for example, GaAs Monolithic Microwave Integrated Circuit (MMIC) amplifiers, are used in a wide variety of applications. In one application, an input microwave signal is split, or divide, using a power splitter, herein after sometimes also referred to herein as a power divider, into two quadrature paths, each path is fed a different amplifier, after power amplification in the amplifiers, the amplifier outputs are recombined in quadrature using a power combiner into a single output signal. There are many types of power dividers and combiners. One of the most popular versions for these applications is a Lange splitter/combiner. The Lange requires tight spacing between signal traces and therefore requires a high technology board fabrication. Often, the splitter/combiner function is actually a separate part and therefore requires interconnects that can drive cost based on assembly yield. An example of two amplifiers that are split and combined using a Lange combiner is shown in FIGS. 1 and 2. All power amplifiers are limited ultimately by the amount of FET periphery that can be reliably fabricated onto one MMIC. This limitation is a balance of thermal considerations and yield. To overcome this issue, power combiners are sometimes used to combine multiple power amplifiers in order to increase the total output power of a unit amplifier (two MMIC amplifier) cell.
There are many different configurations of power combiners and trade-offs to each one. The design trade-offs could be power handling, bandwidth, physical constraints, etc. . . . . The most popular form of power combining power amplifier MMICs is called quadrature combining. The quadrature splitter/combiner arrangement, shown in FIGS. 1 and 2, is used to properly phase combine the overall signal. The input signal is split, as noted above, while delaying one of the power amplifier inputs by 90 degrees (in quadrature). When the signals are re-combined at the output of the power amplifiers, the other signal is delayed by 90 degrees so that both signals combine in phase. This maximizes the overall power combined. The quadrature combiner also has the benefit that any mismatch signal that comes from the power amplifier being terminated in the 4th, or isolation, port, as shown in FIGS. 1 and 2. This is particularly important so that the power amplifier does not load pull the amplifier that drives the input signal. This ultimately helps maximize the total combined output power of the channel.
There are many types of quadrature dividers and combiners including Lange, branch line and overlay, such as, for example, described in U.S. Pat. Nos. 4,185,258 and 6,437,661 where an upper and lower overlaying conductors in regions R1 and R2 as shown in FIG. 2. They are all 4 port devices, where the 4th port is the isolation port. The Lange and branch line combiners are planar combiners, while the overlay is a 3-dimensional layout technique. Lange combiners have very good performance but can be expensive due to the high cost of board fabrication due to tight line to line spacing. Branch line combiners are also used extensively in power amplifier MMIC combining but are often physically large planar structures with very little design flexibility. Overlay combiners are rarely used for power amplifier combining but add the potential for 3-dimensional integration.
In power amplifier combining, cost and size are important considerations to the transmit/receive module designer. The Lange design often is fabricated as separate placeable components due to the cost if the yield of the combiner drives the yield of an overall integrated board. As a component, the Lange drives cost as a separate part as well as adding manufacturing yield issues due to component placement and wire bond interconnect issues. The branch line combiner in itself is fairly low cost technology but is very cumbersome in size which ultimately limits design flexibility and increases overall cost due to overall board size. Overlay combiners can be made in commercial quality boards and can decrease the amount of real estate needed but often have minimal size improvement due to underutilization of the full 3-dimensional capability.
Combining of power amplifier MMICs using quadrature combiners is an established practice within the industry using Lange, branch line and overlay combiners. Some characteristics of Lange couplers are: High technology, high cost boards required for line to line spacing; The yield of this function often drives it to be a separate part which is prone to higher cost and yield fallout due to component placement and interconnect issues; Planar structure uses a lot of space in the x-y plane. Some characteristics of branch line couplers are: Large structure that takes up a lot of space in the x-y plane; very little design flexibility to allow for compact layout. Overlay coupler Industry practice does not fully utilize the z-dimension for compact design